The limited proliferative ability of normal animal cells in culture is widely accepted as a manifestation of aging at the cellular level. Most of the investigations of this phenomenon have been devoted to the study of intracellular changes that occur during in vitro aging. Recent evidence from cell hybridization and cell reconstruction experiments has led to the idea that senescent cells produce a specific substance(s) that pervents DNA synthesis and cell division. We have interpreted the results from these experiments as evidence for the production of a new gene product (or change in the mode of expression of an existing gene product) that results in the loss of DNA synthetic ability (cellular senescence). We propose to identify and characterize the substance(s) responsible for the loss of DNA synthetic ability. This will be accomplished by a combination of cytoplast fusion techniques and cell fractionation and microinjection.